1. Field of the Invention
The present invention relates in general to an RFID interrogator device which performs radio-communications with RFID tags by a backscatter system utilizing absorption and reflection of radio wave and, particularly to an RFID reader/writer or an RFID reader which controls an amplitude level of a reception signal from the RFID tag by a variable gain amplifier.
2. Description of the Related Art
Radio Frequency Identification (RFID) Systems typically include data carriers, such as, e.g. RFID tags, and RFID interrogators, such as, e.g., RFID reader/writer and the radio-communications between the RFID tags and the RFID reader/writers utilize the backscatter system.
RFID tags used in the RFID system may be a passive type or an active type. Passive type RFID tags have no power source whereas active type RFID tags have a power source or battery. Therefore, the passive type RFID tags have a small and portable external size. In the backscatter system, a passive type RFID tag may be used and thus, a RFID reader/writer transmits a continuous wave (CW) that encodes no information to the RFID tag and the RFID tag converts the received continuous wave to DC power as its power source. In addition, the continuous wave can also be used as a carrier signal. When the RFID tag intends to transmit a response to the RFID reader/writer, the RFID tag alternately carries out reflection and absorption of the received continuous wave by varying the impedance of its antenna in response to information to be transmitted. The RFID reader/writer receives this reflected wave from the RFID tag and decodes the reflected wave to obtain information from the RFID tag.
When the RFID reader/writer executes radio-communications with the RFID tag, the RFID reader/writer transmits the continuous wave to the RFID tag and simultaneously receives a reflected wave from the RFID tag because of the above-described backscatter system. Such a simultaneous operation is a typical manner of radio-communications with RFID tags. A linear modulation or keying including an Amplitude-Shift Keying (ASK) and a Phase-Shift Keying (PSK) is commonly used as a modulation system to carry out communications between RFID reader/writers and RFID tags.
On the other hand, a wireless LAN (Local Area Network) system performs linear modulation. With linear modulation, Automatic Gain Control (AGC) is used because, to maintain the linearity, it is required to amplify signals received without any distortion. AGC is a circuit provided in a reception section to maintain a received power constant such that the gain of an amplifier of the reception section is decreased when the RSSI (Received Signal Strength Indicator) is strong and the gain of the amplifier is increased when the RSSI is weak.
Japanese laid-open (kokai) patent application P2003-92561 discloses a radio-communications terminal apparatus that is provided with a reception device performing AGC and is used in a wireless LAN system. Transmission of a radio wave and reception of a radio wave are selectively switched and the RSSI is measured when reception is selected. When the received radio wave goes above a prescribed threshold value of the RSSI, AGC is initiated.
In a digital radio-communications system, such as, e.g., wireless LAN, a radio-communications terminal apparatus transmits radio wave when transmitting and receives radio wave when receiving. Transmission and reception is selectively performed in this system. However, when the RFID reader/writer executes radio-communications with the RFID tag, a backscatter system may be adopted. When the RFID reader/writer receives a response from the RFID tag, the RFID reader/writer transmits radio wave (CW) to the RFID tag and simultaneously receives reflected radio wave (response) from the RFID tag, as described above. In this simultaneous operation by the RFID reader/writer, a short cut of radio wave from the transmission subsystem to the reception subsystem may occur during the reception of the reflected radio wave having a weak power from the RFID tag.
An amount of radio wave short-cutting from the transmission subsystem to the reception subsystem is quite large compared with that of the reflected radio wave from the RFID tag. Thus, it may be difficult to precisely determine a timing at which the reflected radio wave from the RFID tag is firstly received by the RFID reader/writer based on the RSSI. Accordingly, it is not appropriate to adopt a system, utilized in a wireless LAN system that a reception timing of the reflected radio wave is detected based on the RSSI and then AGC is started, to the RFID reader/writer that performs radio-communications with RFID tags.